1. Field of the Invention
Aspects of the invention relate to semiconductor devices, and control ICs for switching power supplies.
2. Description of the Related Art
A control IC for switching power supply is a special IC for controlling individual high breakdown voltage switching transistors. This IC forms a power supply for itself by operating a high-breakdown-voltage switching transistor in an operating state, whereas requires provided with a starting current from a starting circuit at startup. Usually, a starting circuit is integrated on the same semiconductor substrate as a control IC for switching power supply, thereby it is realized to reduce a parts count and to simplify a power supply system.
Starting current is made by rectifying input alternating current signal (AC 100 to 200V), and normally-on type element upstream from starting circuit requires high breakdown voltage of more than 450V, for supplying the starting current to starting circuit. This normally-on type element is made monolithic with control IC for switching power supply, so that it is implemented to a lateral high-breakdown-voltage field effect junction transistor (field effect junction transistor: JFET). Design specifications of switching power supply unit are decided by current driving capability of the element.
When a plug of a switching power supply unit is pulled off the outlet and voltage supply from AC input becomes to be stopped, input voltage of primary side decreases. When the switching power supply unit continues to work in this condition, on-time state of MOSFET (Insulated gate field effect transistor) functioning as switching element gets longer and MOSFET generates heat. To prevent this problem, for switching power supply unit, a brown-out function is provided that stops switching action of power supply when input voltage has decreased.
For realizing the brown-out function, a conventional switching power-supply unit is divided broadly into an external resistor-voltage dividing system or an incorporated IC chip system. External resistor-voltage dividing system means that primary side voltage of power supply is resistor-voltage divided by resistor-voltage dividing circuit connecting two resistors in series externally mounted to control IC for switching power supply (hereinafter referred to as “control IC”). Incorporated IC chip system means that primary side voltage of power supply is resistor-voltage divided by high-breakdown-voltage dividing resistor incorporated in control IC. In an incorporated IC chip system, high-breakdown-voltage dividing resistor (resistor element) is constituted by using voltage resistant structure of high-breakdown-voltage device (starting element).
A switching power supply unit for incorporated IC chip system is proposed that arranges spiral resistor on voltage resistant structure of existing starting element constituting starting circuit in control IC, to realize high-breakdown-voltage dividing resistor. See, for example, Japanese Unexamined Patent Application Publication No. 2008-153636 (also referred to herein as “Patent Document 1”). In the above Patent Document 1, resistors are placed into the spiral along the periphery of starting element, from drain electrode with maximum potential disposed at the center of starting element to source region and gate region surrounding the periphery of drain region so that electric potential falls gradually. Thus resistors are integrated with starting element to allow switching power supply unit to incorporate high breakdown voltage resistor element without providing high breakdown voltage structure newly.
However, when input voltage detecting means adopts resistor voltage dividing circuit with two resistors connected in series, current consumption is always generated in resistor voltage dividing circuit while input voltage is applied. This kind of problem occurs whether resistor voltage dividing circuit is incorporated or externally mounted to semiconductor integrated circuit device. Usually, elevating resistance value of entire resistor voltage dividing circuit suppresses current consumption. However, when incorporating resistor-voltage dividing circuit in semiconductor integrated circuit (IC), the following two problems occur.
The first problem is that starting element is made wide in area and large in size in order to ensure the length of resistor constituting resistor-voltage dividing circuit. The second problem is that variation of resistance value increases because of further lowering impurity dope amount of resistor in order to elevate resistance value of resistor per unit length. Addition of adjusting circuit such as trimming is publicly known as a method to solve these two problems, which has a problem that circuit constitution is made complicated.